Optical fiber laser fusion splicer

ABSTRACT

An optical fiber fusion splicer apparatus (20) comprises a laser power source that produces a laser beam (32) having a laser beam axis (26). The laser power source includes a laser (22), a shutter (28) that controllably blocks and passes the laser beam, and an optical system (30) that expands the diameter of the laser beam. A parabolic mirror (34) has its axis coincident with the laser beam axis (26) and a bore (48) therethrough coincident with the laser beam axis (26). Optical fiber clamps (42, 46) hold the two optical fibers (40, 44) with their axes coincident with the laser beam axis (26) and their ends (62, 64) at the focal point (38) of the parabolic mirror (34). A sensor (82) measures the power reaching the optical fiber ends (62, 64) at the focal point (38) of the parabolic mirror (34), and a controller (72) controls the power level of the laser (22) responsive to the power measured by the sensor. The alignment of the optical fibers (40, 44) is sensed, preferably by a reflective device (74) that measures their internal reflectance or a video camera (68) that images their peripheries, and the optical fiber ends (62, 64) are aligned responsively.

BACKGROUND OF THE INVENTION

This invention relates to optical fibers, and, more particularly, to thesplicing of two lengths of optical fibers to form a single splicedoptical fiber.

Optical fibers are strands of glass fiber processed so that light beamstransmitted through the glass fiber are subject to total internalreflection. A large fraction of the incident intensity of light directedinto the fiber is received at the other end of the fiber, even thoughthe fiber may be hundreds or thousands of meters long. Optical fibershave shown great promise in communications applications, because a highdensity of information may be carried along the fiber. Also, the qualityof the signal is less subject to external interferences of various typesthan are electrical signals carried on metallic wires. Moreover, theglass fibers are light in weight and made from highly plentifulsubstances, such as silicon dioxide.

Glass fibers are typically fabricated by preparing a cylindrical preformof glasses of two different optical indices of refraction, with a coreof one glass inside a casing of a glass of slightly lower refractiveindex, and then processing the preform to a fiber by drawing orextruding. The optical fiber is coated with a polymer layer termed abuffer to protect the glass from scratching or other damage. The opticalfibers and the buffers may be made with varying dimensions, dependingupon their intended use and the manufacturer. As an example of thedimensions, in one configuration the diameter of the glass optical fiberis about 0.002-0.005 inches, and the diameter of the optical fiber plusthe buffer layer is about twice the optical fiber diameter.

For some applications the optical fiber must be many kilometers long andmust have a high degree of optical perfection and strength over thatentire length. Preparation of an optical fiber of that length having nodefects is difficult. It is therefore desirable to have the capabilityto splice two shorter lengths of optical fiber together to form a longeroptical fiber. The need to splice optical fibers also arises when it isnecessary to use a length longer than can be made from a single preform,when an existing length of fiber breaks, or when apparatus such as anamplifier is to be incorporated into a length of fiber.

The optical fiber splice must be accomplished so that there is nosignificant increase in loss of light in the vicinity of the splice. Thespliced fiber must also have a sufficiently high strength to withstandhandling in operations such as winding under tension onto a bobbin, orunwinding from the bobbin at high rates. Additionally, it must bepossible to restore the buffer layer initially on the fibers beingspliced.

A number of techniques for splicing optical fibers are known in the art.For example, U.S. Pat. No. 4,263,495 depicts the use of a laser to heatand fuse the ends of two opposed optical fibers. In this approach, thelaser beam may be directed either perpendicular to the optical fibers,or parallel to the optical fibers and reflected to a focal point by amirror. As such techniques were applied, they were observed to producesplices that were lacking in strength and reproducibility. As aresponse, automated optical fiber splicing control systems such as thatof U.S. Pat. No. 5,016,971 were developed. The automated approach ofU.S. Pat. No. 5,016,971 has significantly improved the ability to spliceoptical fibers in a reproducible manner. However, there remains theopportunity for improving the strength, optical characteristics, andreproducibility of optical fiber splices.

Therefore, there is a continuing need for an improved method forsplicing optical fibers. The improved technique should produce splicedoptical fibers of acceptable strength and optical performance, and havethe ability to provide a continuous buffer coating over the splicedregion. The splicing method should be amenable to accomplishing largenumbers of splices in a reproducible manner. The present inventionfulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for grasping andsplicing lengths of optical fiber together to form a single splicedlength. High quality splices having little loss of light and little lossof strength due to the presence of the splice are produced. Equallyimportantly, the splicing method and apparatus are automaticallycontrolled and yield highly reproducible results when applied in asetting where large numbers of splices must be made on a routine basis.

In accordance with the invention, an optical fiber fusion splicercomprises means for supporting two optical fibers with their ends in analigned facing relation along an axis and means for heating the twooptical fibers uniformly around their circumferences at their facingends. The apparatus further includes means for controlling the powerinput to the means for heating so as to heat the optical fibers at theirfacing ends with the minimum power sufficient to fuse the facing ends.The combination of precisely positioning the ends of the optical fibers,circumferentially evenly heating the ends being spliced, and utilizing aminimum required power to fuse the optical fibers produces excellentquality and reproducibility of the optical fiber splices.

In one embodiment, the heating source is a laser power source thatincludes a laser, a shutter that controllably blocks and passes thelaser beam, a parabolic mirror whose axis is parallel to the axis of theoptical fibers, and an optical system that expands the diameter of thelaser beam before it reaches the parabolic mirror. The laser ispreferably a carbon dioxide laser whose output power is monitored andused for control purposes. There is also preferably a sensor thatmeasures the power of the beam reaching the opticaL fiber ends beingspliced, such as an optical detector that measures the change inluminescence at the optical fiber ends being spliced. A controlleradjusts the beam power responsive to one or both of the sensors. Thepower level of the laser is determined in a series of calibration teststo be the minimum power required to accomplish the fusion splicing. Oncethis power level is determined, the same conditions are used insubsequent splices of similar optical fiber lengths.

The means for supporting the optical fibers preferably includes a firstoptical fiber clamp positioned between the laser and the parabolicmirror, as measured along the beam axis, and a second optical fiberclamp positioned further from the laser than the parabolic mirror, asmeasured along the laser beam axis. The parabolic mirror includes a boretherethrough coincident with the beam axis. Each of the optical fiberlengths whose ends are to be fused and spliced is supported in one ofthe optical fiber clamps, coincident with the laser beam axis. The firstoptical fiber clamp is mounted on a controllable 3-axis manipulatorstage that permits the optical fiber end to be spliced to be positionedvery precisely relative to the other end, which is held fixed in alocation protruding through the bore of the parabolic mirror to thefocus of the mirror.

The optical fibers are monitored and positioned responsively using the3-axis manipulator stage. Two monitoring techniques are used. In one, avideo image of the sides of the optical fibers is obtained with a videocamera. The image may be viewed on a monitor and the manipulator stagemoved responsively. Preferably, the image is processed with patternrecognition techniques to recognize the peripheries of the opticalfibers. The stage is automatically moved responsively to align theperipheries prior to splicing. In a second technique, an optical timedomain reflectometer is used to measure light reflected from defects andsurfaces within the optical fibers. The stage is automatically movedresponsively to minimize the reflected light, thus maximizing lighttransmission.

The present invention also provides a design for the optical fiberclamps that achieves secure clamping and holding of the optical fiberwhile not stressing the optical fiber. The clamp includes a support basehaving a slot therein radially dimensioned to conform to the outerbuffer surface of the optical fiber, and a retainer that slides into theslot with a lower end dimensioned to conform to the outer buffer surfaceof the optical fiber. The optical fiber to be held is inserted into theslot, and the retainer is thereafter inserted into the slot to hold theoptical fiber securely. The slot is precisely located to the outersurface of the clamp to permit the optical fiber to be positionedexactly and reproducibly. A cooperating clamp holder allows the opticalfiber to be moved from place-to-place, and also fromapparatus-to-apparatus, while retained within the clamp. Consequently,there is little likelihood that the optical fiber can be damaged whileheld and moved in the clamp.

After the splicing of the glass optical fibers is complete using theapproach of the invention, the spliced optical fibers must be recoatedwith buffer material. The first important consideration in recoating thenow-spliced optical fiber is to move the optical fiber from the laserfusion apparatus to a recoating unit. Experience has shown that theremoval of the optical fiber from the clamps and movement to therecoating unit can result in damage to or breakage of the optical fiberin the splice region, inasmuch as it is not protected by a buffer layerat that time. The present invention therefore provides a clamp holderthat grasps the two optical fiber clamps and allows them and the splicedoptical fiber lengths to be moved as a rigid unit to the recoatingapparatus. The result of using this clamp holder, which is also usablein other contexts for moving aligned and/or spliced optical fibers, issubstantially improved reliability of the splicing and recoatingoperation.

The present invention provides an important advance in the art ofoptical fiber splicing technology. The approach is highly controllableand reproducible, because the pre-fusion alignment and laser fusionoperations are controlled responsive to automated measurements of theapparatus. The controller, not an operator, determines the alignment andfusion conditions, and the coaxial design ensures circumferentially evenheating of the optical fiber ends as they are fused. Other features andadvantages of the invention will be apparent from the following moredetailed description of the invention, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an optical fiber;

FIG. 2 is a schematic block diagram of the optical fiber splicingapparatus;

FIG. 3 is a side sectional view of the parabolic mirror with opticalfiber lengths in position for splicing;

FIG. 4 is a perspective view of the optical fiber clamp holding anoptical fiber;

FIG. 5 is a perspective view of the optical fiber clamp holder;

FIG. 6 is a perspective view of the recoating apparatus; and

FIG. 7 is a perspective view of a split mold for recoating the opticalfiber with buffer material, with interior features in phantom lines.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a generally cylindrical optical fiber 10, having a core12 of a glass of a selected refractive index and a casing 14 of a glasshaving a slightly lower refractive index. The glass used in the core 12and the casing 14 are of slightly different compositions, and are inmost cases silicon dioxide-based glasses. A buffer coating 16 of apolymer material such as a UV curable acrylate surrounds the opticalfiber 10. By way of illustration and not limitation, for a typical fiberthe cylindrical diameter of the optical fiber 10 (that is, the outerdiameter of the casing 14) is about 0.002-0.005 inches, and thecylindrical diameter of the buffer coating is about twice that of theoptical fiber 10, or about 0.004-0.010 inches. The preferred embodimentof the present invention deals with splicing two of such optical fibers10 in an end-to-end manner, and not the particular structure ormaterials of construction of the optical fibers and buffer coating, andis not so limited.

FIG. 2 depicts a laser fusion apparatus 20. A laser 22, preferably acarbon dioxide laser, produces a laser beam 24 directed along a beamaxis 26. The laser beam 24 first passes through a controllable shutter28 and then through a beam expander 30 that produces an output laserbeam 32 coaxial with the beam axis 26 but of expanded diameter ascompared with the laser beam 24.

A parabolic mirror 34, depicted in greater detail in FIG. 3 and having areflecting surface 36 which is a parabolic surface of revolution orparaboloid, is positioned so that its parabolic axis is coincident withthe beam axis 26. The expanded laser beam 32 is also coincident with thebeam axis 26, so that the laser beam 32 reflects from the parabolicreflecting surface 36 through a focal point 38 of the parabolic mirror34. As depicted by the ray paths of FIG. 3. This arrangement, togetherwith the coaxial alignment and positioning of the ends of the opticalfibers during splicing, ensures circumferentially even heating of theends of the optical fibers during the splicing operation.

A first length of optical fiber 40 to be spliced, from which the bufferlayer 16 has been removed, is supported in a first clamp 42. The firstclamp 42 positions the first length of optical fiber 40 between thelaser 22 and the mirror 34 coincident with the beam axis 26. A secondlength of optical fiber 44 to be spliced, from which the buffer layer 16has been removed, is supported in a second clamp 46. The second clamp 46positions the second length of optical fiber 44 coincident with the beamaxis 26 but further from the laser 22 than the mirror 34 along the beamaxis 26. The mirror 34 has a bore 48 therethrough, coincident with thebeam axis 26, of sufficient diameter so that the second length ofoptical fiber 44 can pass therethrough. The bore 48 preferably has twodiameters, a smaller diameter near the parabolic surface through whichthe optical fiber 44 (with no buffer coating) slides, and a largerdiameter which receives the second clamp 46, as shown in FIG. 3.Alternatively, the second clamp 46 may be supported independently of themirror 34, as shown in FIG. 2. In the preferred embodiment, theparabolic mirror 34 is hinged along its centerline, to permit the mirror34 to be assembled over the second length of optical fiber 44 held inthe second clamp 46.

The clamps 42 and 46 are designed to firmly but gently grasp the buffercoating around a length of optical fiber in a manner such that theoptical fiber can be readily positioned along the beam axis and alsomoved into the proper splicing position relative to the parabolicmirror. A preferred form of the clamps 42 and 46 is shown in FIG. 4. Theclamp 42, 46 includes a elongated base 50 with a cylindrical outersurface 51 and a radial slot 52 extending along the cylindrical axis ofthe base 50. A lower end 54 of the slot 52 is concavely curved with aradius of curvature of about that of the radius of the buffer layer 16of the optical fiber 40, 44, and preferably just slightly larger thanthat of the buffer layer 16 of the optical fiber 40, 44 to be grasped inthe clamp. The lower end 54 is generally concentric with the outersurface 51, and is precisely positioned relative to the outer surface 51of the clamp.

The clamp 42, 46 further includes a retainer 56 with a tongue 58dimensioned to closely fit within the slot 52 when the retainer 56 isassembled to the base 50 by sliding the tongue 58 into the slot 52. Alower end 60 of the tongue 58 is concavely curved with a radius ofcurvature about that of the optical fiber 40, 44, and preferably justslightly larger than that of the buffer of the optical fiber 40, 44 tobe grasped in the clamp 42, 46. When the clamp is assembled with anoptical fiber captured between the lower end 54 of the slot 52 and thelower end 60 of the tongue 58, an outer surface 61 of the retainer 56 isflush with the outer surface 51 of the base 50, so that the outersurfaces 51 and 61 cooperate to form a continuous surface that can bereadily grasped. In the preferred form, the outer surfaces 51 and 61form a generally continuous cylindrical surface when the retainer 56 isassembled to the base 50, with an optical fiber captured between thelower end 54 of the slot 52 and the lower end 60 of the tongue 58.

To grasp an optical fiber 40, 44 in the clamp 42, 46, the retainer 56 isdisassembled from the base 50. The optical fiber 40, 44 is placed intothe slot 52. The retainer 56 is then assembled to the base 50 by slidingthe tongue 58 into the slot 52. The tongue 58 slides downwardly untilthe lower end 60 of the tongue 58 contacts the buffer of the opticalfiber 40, 44, forcing it downwardly to contact the lower end 54 of theslot 52. The optical fiber is thereby held firmly but gently so that itmay be positioned as required, in this case along the beam axis. Thecooperation between the lower ends 54 and 60 holds the optical fiber ata precisely known position relative to the outer surfaces 51 and 61,permitting the optical fiber to be precisely positioned relative to theouter surfaces 51 and 61. The clamping force may be controlled byadjusting the pressure against the surface 61 of the retainer 56.

The clamp of the present approach offers important advantages over priorclamping techniques for optical fibers such as grooved blocks andmagnetic chucks. The optical fiber may be precisely positioned relativeto an externally grasped surface. The clamp is compact, easy to use, anddoes not require external connections such as a vacuum line. The absenceof an external support connection is important, because such hook-upslimit the ease of movement of vacuum-type clamps. In the present clamp,the optical fiber is grasped firmly, so that it cannot move eitherradially or axially in the clamp. This one clamp can therefore be usedfor a variety of applications and situations, with the optical fiberheld securely within the clamp. The optical fiber is inserted into theclamp, and then the clamp is moved from place to place using a clampholder to be discussed subsequently. There is little likelihood that theoptical fiber will be damaged by bending, twisting, or scratching whileheld in the clamp.

By contrast, in prior approaches the optical fiber was typically held bydifferent clamps at each stage of the process, each clamp being tailoredfor its particular stage of the process. While each of the clamps mightbe effective, there was a risk that the optical fiber could be damagedas it was moved from clamp to clamp between operations. This risk isavoided with the present approach, as the optical fiber is not removedfrom the clamps 42, 46 between splicing operations. Although theseclamps 42, 46 were designed for use in the present splicing technique,they have broader use in any application where optical fibers must begrasped firmly but gently.

Returning to FIG. 3, the first length of optical fiber 40 has a firstend 62 to be spliced to a second end 64 of the second length of opticalfiber 44. These ends 62 and 64 lie coaxial with the beam axis 26, andare placed into a close, and preferably lightly touching, facingrelationship at the focal point of the mirror 38.

To effect this positioning, the second end 64 is moved to the properposition at the focal point 38 by manually sliding it along the bore 50.The mirror 34 and the second optical fiber 44 are thereafter heldstationary.

The first clamp 42 holding the first optical fiber 40 is mounted on athree-axis remotely programmable and controllable stage 66. The stage 66permits precisely controllable movement of the first clamp 42 and thencethe first end 62 of the first optical fiber 40 in the two dimensionsperpendicular to the beam axis 26, so that the first optical fiber 40can be positioned exactly coincident with the beam axis 26. The stage 66also permits precisely controllable movement of the first clamp 42 andthence the first end 62 of the first optical fiber 40 toward the mirror34 or away from the mirror 34 in the direction parallel to the beam axis26, to bring the first end 62 to the focal point 38 of the mirror 34.

Two monitoring aids are used to precisely position the first end 62relative to the second end 64. In the first, the region of the focalpoint 38, with the first end 62 on one side and the second end 64 on theother, is monitored by a TV camera 68. The image is viewed in atelevision monitor 70. The image is also provided to amicroprocessor-based controller 72 operating the stage 66. Using themicroprocessor in the controller 72, the image of the two ends 62 and 64and the peripheries of the optical fibers 40 and 44 may be automaticallyanalyzed using conventional pattern recognition techniques so that theperipheries of the optical fiber are identified. The controller 72automatically aligns the peripheries so that the outer boundaries of theoptical fibers 40 and 44 are precisely aligned along the beam axis 26.

In the second monitoring aid, light transmission through the core 12 ofthe optical fiber 10 is monitored using an optical time domainreflectometer (OTDR) 74. The OTDR transmits light through one of theoptical fibers 40 or 44, and measures the intensity of light reflectedfrom defects that may be present in the optical fiber. The OTDRpreferably is used with the shorter of the optical fibers 40 or 44, tomaximize the intensity of the reflected light for analysis, but may beused with the longer of the optical fibers or both optical fibers. Otheroptical monitoring aids may also be used in place of the OTDR, such as apower meter that measures the total light throughput of an optical fiberor a local light injection apparatus that measures the lighttransmission through a segment of the length of the optical fiber.

The use of the OTDR in characterizing and aligning the optical fibershas several important advantages. First, it permits identification,location, and characterization of the optical defects in the opticalfiber, including those at the end to be spliced. Second, it permits theoptical fiber cores 12 to be aligned for the splice, as distinct fromthe peripheries of the optical fibers. In some instances, the cores ofoptical fibers are not perfectly concentric with the outer peripheriesof the casings 14. In those cases, if the visual alignment techniqueusing a television image is used, the optical fiber peripheries will bealigned, but the light-transmitting cores will not be aligned.

The availability of both the visual and OTDR alignment procedures allowsthe user of the optical fiber considerable flexibility in selection ofthe result of the splicing operation. The two techniques are first usedfor screening the optical fibers. The controller determines the positionof the stage 66 for the alignment by each of the two monitoringprocedures. If the positions are different by more than some preselectedamount, it may be concluded that the core is too non-concentric with thecladding. The optical fiber can then be used for some other, lessdemanding application.

After an acceptable optical fiber is found, one or the other of thealignment procedures is used. If the visual alignment procedure is used,the resulting spliced optical fiber will have maximum physical strength,but may not have maximum optical light transmission because the coreswere not perfectly aligned. Conversely, if the OTDR procedure is used,the resulting spliced optical fiber will have maximum lighttransmission, but may not have maximum strength because the peripherieswere not perfectly aligned. The choice of maximum strength, maximumlight transmission, or unacceptability of one or both of the opticalfibers is achieved automatically according to these procedures and theinstructions of the operator.

Once the first end 62 and second end 64 of the optical fiber lengths arepositioned in facing contact at the focal point 38 according to theseprinciples, a pulse of laser energy from the laser 22 is directed to thefocal point 38 by opening the shutter 28 for a predetermined period oftime.

The pulse of laser energy is adjusted to have the minimum power requiredto successfully melt and fuse the ends 62 and 64 together. The use of aminimum power reduces distortion of the glass optical cores of theoptical fibers, and avoids production of melt debris that can distortthe optical properties of the optical fibers and also weaken them. Insome prior art approaches, gas flame or electric arc heating was used tosplice optical fibers. These heating techniques both are inherentlydifficult to control and also can introduce contaminants into the fusedglass. The use of the laser to heat the optical fibers for splicing wasan important advance, but too high a laser power level can cause thevaporization and ejection of silica from the optical fibers. Thesesilica particles redeposit onto the optical fiber as flaws that canreduce the strength of the optical fiber. Minimization of the laserpower minimizes such damage during splicing. The minimization cannot bereproducibly achieved without evenly heating the optical fibers aroundtheir circumference, and positioning the optical fiber ends in areproducible manner, as provided by the present approach.

To determine the proper laser power required and also the laser powersettings and shutter opening duration, calibration tests are performedto splice optical fibers similar to those that will be spliced inproduction. The power produced by the laser 22 is measured by a meter 80operating through a port in the laser. Such measurement capability isroutinely provided in commercial lasers. The power level measured by themeter 80 is provided to the controller 72.

The power reaching the focal point 38 during the splicing operation isalso measured. This measurement is more difficult to obtain. A lightintensity sensor 82 is positioned to view the ends 62 and 64 at thefocal point 38 during the splicing event. The intensity of the light isdetected by the sensor 82, and the output of the sensor 82 is amplifiedby an amplifier 84 and provided to the controller 72. It will beappreciated that the light intensity is not a direct measure of power,but is a power-dependent parameter that can serve as the basis for thecontrol of the laser source power settings. Thus, during the calibrationtesting the relative positioning of the optical fiber ends 62 and 64 isdetermined from the visual image on the monitor 70 and correlated withthe position of the stage 66 using the image processing capability ofthe controller 72. The laser power applied to the ends during splicingis measured both as laser output power and as the emitted light from theregion of the splice.

The optical fibers spliced together during calibration are evaluated asto their mechanical and optical properties, and from the correlationwith the measured splicing parameters an optimal set of splicingparameters is selected. The splicing parameters include both thepositioning and the power information measured during splicing. With thereproducibility afforded by the use of the microcomputer-basedcontroller 72, these conditions can be reproduced by controlling thepositioning of the stage 66 and the power applied to the laser 22through a laser amplifier 86, both of which are established by thecontroller 72.

After the ends of the optical fibers 40 and 44 are spliced, the barelength of spliced glass must be recoated with buffer material and cured.In the past, it has been common practice to remove the spliced lengthfrom the splicing apparatus (of different type than that of the presentinvention) and to gently move it to a coating apparatus. It has now beenfound that, no matter how gently this movement is accomplished, the bendor twist stressing of the optical fiber along the bare, uncoated lengthcan lead to imperfections and possible premature later failure in someinstances.

To avoid this bending or twisting of the spliced optical fiber duringmovement from the laser splicing apparatus to the recoating apparatus, aclamp holder 90, shown in FIG. 5, has been designed. The clamp holder 90includes two grasping arms 92 and 94. These arms 92 and 94 are spacedapart the proper distance to grasp the two clamps 42 and 46,respectively, in their normal position in the apparatus 20. The clamps42 and 46, with the now-spliced optical fibers 40 and 42 in place withinthe clamps, are grasped by the arms 92 and 94 and moved as a unit to therecoating apparatus.

Each of the arms 92 and 94 includes a respective facing pair of graspingblocks 96 and 98 that are pivotably mounted to a grasping block base100. A spring (not shown) biases the two blocks 96 and 98 of each pairtoward each other. The grasping blocks 96 and 98 are dimensioned tofirmly grasp the sides of the clamps 42 and 44. Respective downwardlyextending locating pins 102 and 104 are also mounted from each of thebases 100. The purpose of these locating pins will be discussedsubsequently.

The arms 92 and 94 are supported from a handle 106. A movable handlegrip 108 operates a linkage extending inside each arm to the graspingblocks 96 and 98. When the handle grip 108 is operated, the graspingblocks 96 and 98 are pivoted apart against the spring biasing force sothat the arms can be lowered over the clamps 42 and 46. When the handlegrip 108 is released, the grasping blocks again pivot closed, capturingthe clamps 42 and 46. The clamps 42 and 46, together with the splicedoptical fiber lengths 40 and 44, are then moved as a single rigid unitthat does not bend or twist the optical fiber lengths 40 and 44 duringtransport. On the other hand, in some cases it may be desirable toprestress the optical fibers 40 and 44 in tension during recoating, andthe clamps 42 and 46 and clamp holder 90 permit the application of acontrollable pretensioning. The clamps 42 and 46 are released by theinverse operation of the handle grip. The clamp holder 90 was developedspecifically for use in the splicing operation discussed herein, but canalso be used for other operations involving movement of preciselypositioned and/or aligned optical fibers and optical fiber devices.

Recoating of the optical fiber is accomplished with an apparatus 112illustrated in FIG. 6. The clamps 42, 46 rest in cradles 114 and 116,respectively. Positioning of the clamps into the cradles is aided bylocating the pins 102 and 104 of the clamp holder 90 into locating slots118 and 120 of the apparatus 112.

Once the clamps 42 and 46, and thence the optical fiber lengths arepositioned, the optical fiber is recoated with a buffer layer in theregions between the previously unstripped coating material, so that theregion of the splice is fully protected against scratching. A split mold130 suitable for casting a flowable polymer around the optical fibers40, 44 is illustrated in FIG. 7. The mold 130, when assembled around theoptical fibers 40, 44, rests on a second set of cradles 122 and 124around the uncoated portion of the optical fiber.

The mold 130 includes a bottom 132 and a mating top 134, both made of amaterial transparent to ultraviolet light, such as plexiglass. Thebottom 132 and top 134 have semicircular recesses therein along thefacing surfaces so that, when the bottom 132 and top 134 are assembledtogether, a cylindrical central cavity 136 is formed. The bottom 132 andtop 134 are assembled over the optical fibers 40, 44 so that the portionof the optical fibers from which the buffer layer 16 has not beenremoved is positioned at the ends of the mold 130 as seals. The centralportion 138, from which the buffer layer 16 has been removed for thelaser splicing operation, lies within the interior of the cavity 136.External access to the interior of the cavity 136 is provided through aport 140.

To perform the buffer recoating, a layer of a release agent such aspolytetrafluoroethylene (teflon) may be sprayed on the matching faces ofthe bottom 132 and top 134. The bottom 132 of the mold 130 is placedonto its cradle 122 or 124. The clamps 42 and 46 are placed into theircradles 114 and 116, respectively, so that the central portion 138 lieswithin the cavity 136 of the as-yet open mold. The remainder of thecavity 136 is filled by injection through a side port 140 with theflowable, ultraviolet curable polymer that polymerizes to become thebuffer coating, in this case a UV curable acrylate. The polymer is curedby directing ultraviolet light of wavelength appropriate to the polymerinto the previously uncured polymer, through the transparent walls ofthe plexiglass mold. The preferred ultraviolet light source is a mercurylamp with a strong UV output at about 350 nanometers wavelength, andcuring is accomplished in a total of about 10 seconds.

The spliced and recoated optical fiber is removed from the recoatingapparatus 112 and the clamps 42 and 46 and inspected. In a typicalinstance, the spliced region will be indistinguishable from theremainder of the length of the optical fiber, except possible for aslight color difference in the buffer material.

A number of optical fibers have been spliced by this approach andmeasured both as to optical performance and mechanical strength, aftersplicing.

In a first series of tests, 12 splices of optical fiber were made. (Athirteenth splice was prepared, but it was found through use of the OTDRthat the optical fiber had a serious defect well spaced from the regionof the splice, and it was dropped from the study.) The 12 splices weremeasured to have an optical attenuation to 1.3 micrometer wavelengthradiation of 0.089+/-0.089 dB.

In a second series of test, 5 splices of optical fibers were prepared.No attenuation due to the splice was measured at 1.3 micrometerwavelength.

Mechanical data for about 100 spliced optical fibers was taken byloading the spliced optical fibers to tensile failure in an Instrontesting machine. The failure strengths were measured in excess of330,000 pounds per square inch (psi), and many were in excess of 400,000psi.

In a production test of the approach of the invention, 5 splices weremade in a fully automated manner with no operator intervention. Themeasured attenuations at 1.3 micrometer wavelength were less than 0.3 dBfor each splice, and the strengths were in excess of 300,000 psi in eachcase.

The present invention provides an important advance in the art ofsplicing optical fibers. It permits good splices to be prepared in anautomated, reproducible manner that is well suited for commercialoperations that are not dependent upon operator skill and patience.Although particular embodiments of the invention have been described indetail for purposes of illustration, various modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited except as by theappended claims.

What is claimed is:
 1. An optical fiber fusion splicer, comprising:meansfor supporting two optical fibers with their ends in an aligned facingrelation along an axis; means for monitoring the positioning of saidfibers in the aligned facing relation; means for optically heating thetwo optical fibers uniformly around their circumferences at their facingends, wherein said means for heating includes:a controllable opticalsource for producing a beam coincident with said axis, and means foroptically measuring the power applied to the facing ends of said twooptical fibers, and means for controlling the power input to the meansfor heating so as to heat the optical fibers at their facing ends withthe minimum power sufficient to fuse the facing ends.
 2. The opticalfiber fusion splicer of claim 1, wherein the means for heating includesalaser power source that produces a laser beam coincident with the axis,and a parabolic mirror having a parabolic mirror axis coincident withthe axis along which the optical fibers are positioned.
 3. The opticalfiber fusion splicer of claim 1, wherein the means for heating includesalaser, a shutter that controllably blocks and passes the laser beam, andan optical system that expands the diameter of the laser beam.
 4. Theoptical fiber fusion splicer of claim 1, wherein the means for heatingincludes a carbon dioxide laser.
 5. The optical fiber fusion splicer ofclaim 1, wherein the means for supporting includesmeans for reflectivelysensing light transmission through at least one of the optical fibers.6. The optical fiber fusion splicer of claim 5, wherein the means forsupporting further includesmeans for positioning the optical fibersresponsive to the means for reflectively sensing.
 7. The optical fiberfusion splicer of claim 1, wherein the means for supportingincludesmeans for imaging the peripheries of the optical fibers.
 8. Theoptical fiber fusion splicer of claim 7, wherein the means forsupporting further includesmeans for positioning the optical fibersresponsive to the means for imaging.
 9. The optical fiber fusion splicerof claim 1, wherein the means for heating includesa laser, a power meterthat measures the output power of the laser, means for measuring thepower applied to the ends of the optical fibers to be spliced, and meansfor controlling the output power of the laser responsive to the powermeter and the means for measuring.
 10. The optical fiber fusion splicerof claim 1, wherein the means for supporting includesa first opticalfiber clamp that holds one of the optical fibers, and a second opticalfiber clamp that holds the other of the optical fibers.
 11. The opticalfiber fusion splicer of claim 10, wherein at least one of the opticalfiber clamps comprisesa support base having a groove therein dimensionedto conform to the optical fiber, and a retainer that fits into thegroove and has a contact end dimensioned to conform to the opticalfiber.
 12. The optical fiber fusion splicer of claim 10, furtherincludingmeans for controllably moving the first optical fiber clamp inthree axes.
 13. The optical fiber fusion splicer of claim 1, furtherincludingmeans for recoating the spliced optical fibers with buffermaterial.
 14. An optical fiber fusion splicer, comprising:a laser powersource that produces a laser beam having a laser beam axis, the laserpower source including a laser, a shutter that controllably blocks andpasses the laser beam, and an optical system that expands the diameterof the laser beam; a parabolic mirror having its axis coincident withthe laser beam axis and having a bore therethrough coincident with thelaser beam axis; a first optical fiber clamp positioned between thelaser and the parabolic mirror, as measured along the laser beam axis,the first optical fiber clamp holding and positioning a first opticalfiber to be spliced with its axis coincident with the laser beam axisand with the first end of the first optical fiber to be spliced at thefocus of the parabolic mirror; a second optical fiber clamp positionedfurther from the laser than the parabolic mirror, as measured along thebeam axis, the second optical fiber clamp holding and positioning asecond optical fiber to be spliced with its axis coincident with thelaser beam axis and with the second end of the second optical fiber tobe spliced at the focus of the parabolic mirror; a sensor that measuresthe power reaching the first and second optical fiber ends at the focusof the parabolic mirror; and a controller that controls the power levelof the laser beam responsive to the power measured by the sensor. 15.The optical fiber fusion splicer of claim 14, wherein at least one ofthe optical fiber clamps comprisesa support base having a groove thereindimensioned to conform to the optical fiber, and a retainer that fitsinto the groove and has a contact end dimensioned to conform to theoptical fiber.
 16. The optical fiber fusion splicer of claim 14, furtherincludinga clamp holder that fixedly positions the first and secondoptical fiber clamps in a preestablished relation relative to eachother.
 17. The optical fiber fusion splicer of claim 14, furtherincludingan instrument that reflectively measures the presence ofdefects within at least one of the optical fibers.
 18. The optical fiberfusion splicer of claim 14, further includinga remotely controllablestage that controllably moves the first optical fiber clamp along threeaxes, and a video sensor that images the peripheries of the opticalfibers.
 19. The optical fiber fusion splicer of claim 18, wherein thecontroller receives the images of the video sensor and performs patternrecognition of those images, and thereafter responsively commands theremotely controllable stage to position the first optical fiber clamp.20. A method for splicing together the ends of two optical fibers,comprising the steps ofsupporting two optical fibers with their ends inan aligned facing relation along an axis; applying controlled power tothe facing ends of said two optical fibers to heat the two opticalfibers uniformly around their circumferences at their facing ends;measuring the power applied to the facing ends of said two opticalfibers, and controlling the power input during the step of heating so asto heat the optical fibers at their facing ends with the minimum powersufficient to fuse the facing ends.
 21. The method of claim 20, whereinthe step of heating is accomplished utilizing a laser.
 22. The method ofclaim 20, wherein the step of controlling includes the stepsofmonitoring with a video camera the ends of the two optical fibers tobe spliced, and moving the optical fibers responsive to the images ofthe video camera to align the ends.
 23. The method of claim 20, whereinthe step of controlling includes the steps ofmonitoring light reflectedfrom surfaces and defects within at least one of the optical fibers, andmoving the optical fibers responsive to the intensity of the reflectedlight to align the ends.